1. Technical Field of the Invention
This invention relates to devices and systems for measuring the angular displacement or rate of displacement of a point or area of interest of a structure, such as the amount of twist of a wing of an aircraft model in a wind tunnel.
2. Description of the Prior Art
Although as will be apparent, and as is discussed below, the invention is not limited to such an application, one important application of the invention is in providing remote, dynamic measurement of the angular rotation (twist) of a wind tunnel model, or other models or structures, under load. Prior art techniques have often relied on the use of strain gages which, in use, are attached to the areas of interest of the structure under load and which require accurate calibration. Further, the load distribution on the model or other structure will affect the twist or other angular displacement at the locations of interest in a complex manner which cannot be fully determined, thereby giving rise to uncertainties in the accuracy of the measurements produced.
Another common technique uses a series of photographs taken under each test condition. This technique is of limited accuracy and requires laborious setup procedures and time-consuming analysis. Further, this technique obviously cannot provide real-time information.
Another technique of particular interest here is that disclosed in U.S. Pat. No. 4,688,934 (Clark). This patent discloses a rotating polarizer angle sensing system for measuring aeroelastic deformation of an aircraft wing in flight or in a wind tunnel which uses light from a linearly polarized light source. A carrier beam is produced, a small portion of which is reflected through a linear polarizer to a first photodetector for use as a reference and the remainder of which is transmitted to a retroreflector target located on the wing. The target reflects the light back to a second photodetector which produces an electrical output signal for comparison with the electrical output signal produced by the reference photodetector. An angle measurement signal is based on this comparison.
A serious disadvantage of the measurement system disclosed in the Clark et al patent concerns the use therein of an external light source and retroreflector in generating the measurement signals. In this regard, because the light must be transmitted from the source to the target, e.g., to the wind tunnel model, and reflected back from the target, anything in the light path that interferes with the light transmission such as fog, dust or the like will adversely affect the measurement. This may not be a problem in a wind tunnel environment but certainly would impair any measurements made outside of such a protected environment, particularly those made over any substantial distance such as measurements of the flutter of a wing of an actual aircraft or measurements made over distance in connection with a remote structure such as a bridge or the like.
Further, vibration of a structure such as a bridge will produce vibrations of the retroreflective target and this presents special problems in a system wherein light is both transmitted to and received from the retroreflective target.
Other patented devices of possible interest here include those disclosed in U.S. Pat. Nos. 4,173,412 (Ramsay et al); 4,321,831 (Tomlinson et al); 4,420,251 (James et al); 4,321,831 (Meltz et al); 4,788,868 (Wilk); and 4,806,012 (Meltz et al). Briefly considering these patents, the Meltz patents both disclose a distributed, spatially resolving optical fiber strain gauge wherein the core of the optical fiber is written with periodic grating patterns effective for transmitting and receiving light injected into the core. Strain is measured by determining spectral shifts between the transmitted and reflected light. The Ramsay et al patent discloses a strain sensor for measuring the magnitude of the strain in a single mode optical fiber using polarized light and a liner polarization analyzer. The Tomlinson et al patent discloses an apparatus for making optical pressure measurements wherein fiber optics are used to transport a polarized beam to a sensing head and a polarization analyzer is employed. The James et al patent discloses an optical sensor for determining fatigue, vibration, flex and the like at a monitored area using polarized light and an optical detector that is responsive to the phase or polarization angle of a reflected light signal. The Wilk patent discloses a strain measurement apparatus which measures the relative movement of one end of a cylindrical member with respect to the other end and which uses optical fibers.